“Wrinkle finishes,” as used in the present specification, are reticulated, i.e., made up of a pattern of raised veins across the surface, and are exemplified by interconnected star-burst patterns, mosaics, and by patterns similar to that of an alligator or an elephant hide. In contrast to wrinkle finishes, “texture finishes” comprise uneven patterns of raised bumps, and nodules in rough surfaces which resemble sand paper and which may include broken portions of a wrinkle finish. A texture finish usually contains pinholes and surface defects resulting from the failure of the finish to cover an entire surface.
Wrinkle finishes are desired in many applications and are commonly applied to office equipment such as typewriters, staplers, dictating equipment, file cabinets, tool boxes, and the like. In addition to being aesthetically pleasing, these finishes have certain utilitarian functions: they hide the surface defects of a substrate even when applied in thin films and, having a low gloss, they do not show scratches. Wrinkle finishes have been applied from powder coatings in which the surface sets up first and, when the rest of the coating cures, the resulting shrinkages deform the previously set surface and cause a fine wrinkle pattern to develop on the surface of the coating. However, the art has made little mention of heat resistant powder coating wrinkle finishes, e.g. for use in coating automobile or motorcycle engine covers, parts, exhaust systems and manifolds, stoves, stacks, stovepipes, grilles, cooking utensils, heat exchangers and fireplaces. Further, finish control problems, i.e. where texture finishes undesirably result, have plagued the development of wrinkle finishes from powder coatings.
U.S. Patent Application Publication 2002/0028879A1, to Chasser et al., discloses powder coatings and methods for forming a wrinkle finish using carbamate functional polymers mixed with a curing agent. As exemplary curing agents, Chasser et al. disclose silicon-containing resins which confer added heat resistance to a powder coating. However, the carbamate group-containing polymers of Chasser et al. are very highly functionalized, having a carbamate equivalent weight within the range of 15 to 150 and a number average molecular weight of from 500 to 20,000. The highly functionalized carbamate-functional polymers of Chasser et al. tend to cure too quickly, often giving a bumpy textured finish rather than a controlled and predictable wrinkle finish.
Heat-resistant powder coating finishes comprising silicone resins are known, albeit without disclosure of a way to form a wrinkle finish using them. Chasser et al. do not provide powder coatings which comprise a resin consisting essentially of silicone resin, i.e. as the only resin present, because silicone resins are known to adhere poorly to most substrates. Chasser et al. attempt to remedy these deficiencies by adding highly functionalized carbamate group-containing resins to the powder coatings which, as discussed above, react too quickly to allow for the achievement of a controlled wrinkle finish.
In overcoming the above-mentioned deficiencies in powder coatings, the present inventors have discovered a powder coating which results in a surprisingly consistent wrinkle finish that is highly heat resistant at 350° F. (285° C.) or higher, preferably from 550° F. to 650° F. In accordance with the present invention, a powder coating provides a heat resistant wrinkle finish which avoids the finish control problems of highly functionalized resins.